1. Field of the Invention
The present invention relates generally to processes for semiconductor manufacturing and more particularly to the area of optical lithography. Especially methods for the determination of focal plane deviation (FPD) associated with photolithographic projection systems.
2. Description of the Related Art
In order to produce sub-wavelength semiconductor patterned features (transistors, gates) with very tight process specifications lithography engineers continuously monitor focus during and after the optical lithography process. The ability to produce sub-wavelength features can often be determined by considering the rather simple (3-beam) Rayleigh scaling Resolution (R) (λ/2NA) and Depth-of-Focus (DoF) equations, ˜λ/2NA and ˜λ/2NA2. These coupled equations stress the inverse relationship between resolution and DoF based on the exposure wavelength (λ) and numerical aperture (NA). The semiconductor industry's requirement to produce smaller critical features over time has forced semiconductor manufacturers and lithography tool vendors to produce higher NA lithography systems (steppers or scanners) using exposure sources at smaller wavelengths (for example, 248 nm). The ability to control focus during the lithography process becomes more difficult as the DoF becomes smaller simply because image fidelity degrades quickly with focal changes. Poor lithographic imaging and poor product yields cause semiconductor manufacturing costs to rise and technology ramp to slow. Semiconductor lithographers have discovered creative reticle enhancement techniques (RETs) and other optical techniques to increase the useable DoF. See, for example, “The Attenuated Phase Shift Mask”, B. Lin, FLEX like “Method and Apparatus for Enhancing the Focus Latitude in Lithography”, Pei-Yang Yan, U.S. Pat. No. 5,303,002 issued Apr. 12, 1994. Despite these efforts, the problem remains. Therefore, it is important to monitor focus during photolithographic processing and develop new methods for focus control. Typically, focus error across a scanner field can be attributed to the following three terms: (1) wafer and reticle non-flatness, (2) dynamic wafer/reticle stage error, and (3) static or dynamic lens field curvature.
For a photolithographic scanner, dynamic lens field curvature varies in the cross scan direction (x) in rather complex ways. While many methods exist for determining and monitoring focal plane deviation (FPD) and best focus by field position for photolithographic exposure tools, these do not account for wafer non-flatness and scanner dynamics, independently of the lithographic process. See, for example, “Distinguishing Dose from Defocus for In-Line Lithography Control”, C. Ausschnitt, SPIE, Vol. 3677, pp. 140-147, 1999; “Latent Image Metrology for Production Wafer Steppers”, P. Kirksen et al., SPIE, Vol. 2440, pp. 701-711, 1995; “Controlling Focal Plane Tilt”, S. Hsu et al., Semiconductor International, Apr. 1, 1998 (available on-line as of February 2004 at the URL of http://www.reed-lectronics.com/semiconductor/article/CA177590?pubdate=4%2F1%2F1999&spacedesc=webex); “Apparatus, Method of Measurement and Method of Data Analysis for Correction of Optical System”, A. Smith et al., U.S. Pat. No. 5,828,455 issued Oct. 7, 1998; and “Apparatus, Method of Measurement and Method of Data Analysis for Correction of Optical System”, A. Smith et al., U.S. Pat. No. 5,978,085 issued Nov. 2, 1999.
Lithography Process Control and Monitoring
A typical microelectronic device or circuit includes many (˜20) levels or pattern layers. The fidelity and placement of patterned features on critical levels is often difficult to control. Lithographers typically use the following metrics to measure the success (or failure) of the lithographic patterning process: (1) critical dimension (CD), a measure of the critical device feature, (2) overlay error or feature position, as described above, and (3) side wall angle (SWA), the shape of side walls of the critical features. Each of these metrics is illustrated in FIG. 1a. These three metrics are typically measured using a scanning electron microscope (CD-SEM) or optical metrology tool (overlay tool). See, for example, “Quaestor Q7 Brochure”, Bio-Rad Semiconductor Systems. Optical metrology tools are less expensive to operate as compared with a CD-SEM and are often used for process control (focus and exposure) applications, as well as overlay monitoring. See, for example, “Distinguishing Dose from Defocus for In-Line Lithography Control”, supra.
Most overlay and CD measurements are made on silicon product wafers after each photolithographic process, prior to final etch. Product wafers cannot be etched until the photoresist features are imaged properly and meet the target process specifications (CD and SWA within process limits). Lithographic process engineers rely heavily on exposure tool alignment and focusing calibration procedures to help insure that the scanner is aligning and focusing images properly; poor focus monitoring techniques corrupt the scanner calibration database and degrades lithographic tool performance. See, for example, “193 Step nm and Scan Lithography”, G. Davies et al., Semi Tech Symposium, Japan, 1998 and “Using the Focus Monitor Test Mask to Characterize Lithographic Performance”, R. Mih et al., SPIE, Vol. 2440, pp. 657-666, 1995. In addition, lack of information concerning the magnitude of fixed errors (aberrations) corrupts process control and overlay modeling routines that try to model-out systematic and random lithographic error.
Over the past 30 years, the semiconductor industry has continued to produce faster (via smaller critical features) and more complex (greater functionality, dense patterning) circuits, year after year. The push to smaller feature sizes is gated by many physical limitations. As the critical dimensions of semiconductor devices approach 50 nm, the usable DoF will approach 100 nm. See, for example, “2001 ITRS Roadmap”, SEMATECH, pp. 1-21, 2001. Continued advances in lithography equipment (higher NA systems, smaller wavelength exposure sources), RET, resist processing, and automated process (focus and exposure) control techniques will likely become more difficult and remain critical. See, for example, “2001 ITRS Roadmap”, supra. Finally, while FPD measurement on a routine basis is important for lithographic process control, separating the FPD into correctable and non-correctable components is important for assessing the capability limits of advanced process control schemes.
Mathematical Description of Focusing Contributions
Dynamic lens field curvature (ZDLC) is that portion of total defocus due to the lens alone. It can be expressed as a weighted integral of the static lens field curvature as:
                              ZDLC          ⁡                      (            x            )                          =                              ∫                                          -                SH                            2                                      SH              /              2                                ⁢                                          ⁢                                    ⅆ              y                        *                          wt              ⁡                              (                y                )                                      ⁢                          ZSLC              ⁡                              (                                  x                  ,                  y                                )                                                                        Equation        ⁢                                  ⁢                  (          1          )                    where:    ZSLC (x,y)=static lens field curvature over the scanned lens slot    y=scan direction coordinate    wt(y)=weighting function ˜I (y), the intensity across the slot for DUV resists but is generally different for I-line resists in particular it will depend on the scan direction being more heavily weighted on the side of the slot height (+y or −y) where the scan begins.    SH=scanner slot height (FIG. 1b)It should be noted that different illumination settings (conventional, strong annular, quadrupole, etc.) will generally have different illumination profiles 1(y) and therefore will have different ZDLC profiles.
While one could determine ZDLC by measuring or otherwise knowing wt(y) and ZSLC(x,y), it would be advantageous to directly determine ZDLC(x).
FPD: There are a number of methods that with greater or lesser accuracy measure defocus or focal plane deviation (FPD) over an exposure field. In general terms, each of these techniques estimate the focal error across the field using a variety of special reticle patterns (focusing fiducials, FF). In addition, some of these methods utilize the stepper or scanner wafer stage leveling and positioning system and/or optical alignment system to aide in the determination of FPD. See, for example, “Twin Scan 1100 Product Literature”, ASML. The term “FPD” is a rather general term describing the complete focus error associated with the photolithographic stepper or scanner-deviations from the focal plane in reference to the wafer surface. Among other things, FPD can be caused by lens tilt, stage/reticle tilt, reticle bow, lens field curvature, and stage synchronization error. FIG. 3a shows a generic photolithographic leveling system. FIGS. 3b and 3c illustrate some common reticle patterns (e.g., the IBM Corporation Phase Shift Focus Monitor (PSFM), and the ASML FOCAL alignment mark) that are used to determine FPD for both steppers and scanners. Typically, FPD calibration/monitoring is performed daily or at least weekly to ensure that the stepper or scanner is operating within design limits (verifying the focus system works, the stage is level, etc.). While both techniques are widely accepted both techniques require complex calibrations to be performed at each field point. See, for example, “Detailed Study of a Phase-Shift Focus Monitor”, G. Pugh et al., SPIE, Vol. 2440, pp. 690-700, 1995; and “Latent Image Metrology for Production Wafer Steppers”, supra.
These FPD prior art methods are listed Table 1 below:
TABLE 1MethodMeasurement TypeCommentISI (See, for example, U.S. Pat. No.AbsoluteExtremely accurate.5,828,455, supra and U.S. Pat. No.5,978,085, supra)FOCAL (See, for example, “FOCAL,RelativePublished version claimsP. Dirksen et al., SPIE, Vol. 2440, p.high absolute accuracy,701, 1995)resolution averaging inpracticeIBM focus monitor (See, for example,AbsoluteRequires ‘calibration’. It“Optical Focus Phase Shift Testis very processPattern, Monitoring System andindependent.Process”, T. Brunner et al., U.S.Pat. No. 5,300,786, Apr. 5, 1994)Schnitzl (See, for example,Relative with oneComplex calibration,“Distinguishing Dose from Defocusexposurevarying target sensitivity,for In-Line Lithography Control”,sign of focus ambiguous.supra)TIS (See, for example, “193 Step andRelativeRelies on wafer Z-stage,Scan Lithography”, supra and “Twinaccuracy/repeat.Scan 1100 Product Literature”, supra)
ISI (Litel): A method for determining the aberrations of an optical system is described in U.S. Pat. No. 5,828,455 to A. Smith supra and U.S. Pat. No. 5,978,085 to A. Smith supra. In these descriptions, a special reticle is used to determine the Zernike coefficients for photolithographic steppers and scanners. Knowing the wavefront aberration (Zernike coefficients and the associated polynomial) associated with the exit pupil of the projection system includes information about the lens field curvature or focus (Zernike coefficient a4, for example). A special reticle and a self-referencing technique are used to rapidly identify FPD to a high degree of accuracy and determine focusing errors to ˜5 nm, even in the presence of scanner noise. This method automatically determines lens field curvature information for both static and dynamic exposure tools (steppers and scanners).
PSFM: A method (Phase Shift Focus Monitor) described in U.S. Pat. No. 5,300,786 supra, can be used to determine and monitor the focal plane deviation (FPD) associated with the lithographic process. More information can be found in the document “Detailed Study of a Phase-Shift Focus Monitor”, referred to above. In general, an alternating PSM with phase close to 90° possesses unusual optical properties that can be exploited to measure focus errors. See, for example, “Quantitative Stepper Metrology Using the Focus Monitor Test Mask”, T. Brunner et al., SPIE, Vol. 2197, pp. 541-549 and “Using the Focus Monitor Test Mask to Characterize Lithographic Performance”, supra. It is generally possible to design a “box-in-box” overlay target using a phase shift mask pattern (called here a focusing fiducial; see FIGS. 3b and 3c), in which the measured overlay error is proportional to the focus error (see FIG. 5). Focal plane non-flatness is then determined by measuring the focusing fiducials across the lens field. Astigmatism information appears as differences between the delta-X overlay error and the delta-Y overlay error measurement. This technology has also been used for assessing variations in focus across the wafer due to lens heating, misfocusing near the edge of the wafer, and chuck/stage non-flatness. One major drawback with the PSFM method is that a fairly elaborate calibration procedure (focus offset vs. overlay shift for each field point) is required before it can be used, as the PSFM technique is rather sensitive to the source-sigma (i.e., NA-source/NA-objective) that varies from process to process. Additional PSM techniques, such as those described in “Focus Monitor for Alternating Phase Shift Masks”, L. Liebmann et al., U.S. Pat. No. 5,936,738 issued Aug. 10, 1999, are used in a similar way. While the PSFM method provides an FPD map across a scanner or stepper field it does not provide a method for determining the dynamic lens field curvature independent of wafer height variation in the presence of stage synchronization error. See, for example, “Comprehensive Focus-Overlay-CD Correlation to Identify Photolithographic Performance”, Dusa et al., SPIE, Vol. 2726-2729, 1996.
FOCAL: A method (FOCAL—Focus determination using stepper alignment system) described by P. Dirksen et al. in “FOCAL”, supra, specifies a focusing fiducial that can be used to find FPD and astigmatism across the exposure field (lens). FOCAL alignment marks (focusing fiducials) consist of modified wafer alignment marks that are measured using the stepper wafer alignment subsystem. See “Latent Image Metrology for Production Wafer Steppers” by P. Dirksen et al., SPIE Vol. 2440, 1995 at pp. 701-711. Defocus of the tool results in an apparent shift of the center of the alignment mark relative to that of the ‘best focus’ position. The FOCAL technique makes use of the exposure tool's alignment mechanism and therefore requires that the stepper or scanner be off-line for the length of the measurement sequence. FOCAL marks are sensitive to exposure and sigma like the PSFM method; however, since fiducial response is a function of pitch, the target features are less dependent upon reticle error. Furthermore, the FOCAL data (focus vs. overlay error) must be calibrated for every point in the exposure field similar to phase-shift monitors (typically at 121 points across an exposure field). Now, it is possible to use FOCAL to separate out lens tilt and astigmatism from dynamic FPD maps and provide a dynamic focal plane map, but wafer height variation and stage synchronization errors would still be included in the result. See, for example, “193 Step and Scan Lithography”, supra, and “Comprehensive Focus-Overlay-CD Correlation to Identify Photolithographic Performance”, Dusa et al., SPIE, Vol. 2726-2729, 1996.
Schnitzl Targets: A method described in “Distinguishing Dose from Defocus for In-Line Lithography Control”, supra, makes use of line-end shortening effects to decouple focus drift from exposure drift on semiconductor product wafers. FIG. 3c shows a typical pair of Schnitzl targets (focusing fiducials). It is widely known that resist line-ends (FIG. 3c) are very sensitive (exhibit greater line-end shortening) to both focus and exposure drifts; the effect is further enhanced as the lithographic process is pushed near performance limit of the scanner tool (˜λ/2NA). Using the Schnitzl targets and a fairly elaborate method of calibration (CD-SEM measurements and a coupled system of equations), Ausschnitt describes a method that can determine the magnitude of focus drift on product wafers using one or more exposures in the presence of exposure drift. Since changes in focus and exposure can produce similar changes in the critical dimension (CD), the Schnitzl method is useful for day-to-day process monitoring because it eliminates the need to constantly perform focus and exposure experiments (FEM—a Focus Exposure Matrix) in-between production runs. In addition, the method uses fast and accurate optical overlay tools to measure the Schnitzl patterns (in several forms, CD targets or Overlay targets, FIG. 3b) after wafer processing, this saves monitoring costs because optical overlay tools are less expensive to operate as compared with a CD-SEM. While decoupling focus drift from exposure drift is useful for process monitoring, the method in its present form requires two exposures at different focus settings to determine the absolute focal drift (direction). Performing extra exposures during production runs is very costly. In addition, since the initial Schnitzl target calibration procedure depends on a number of lithographic tool settings (line size, pitch, sigma, NA) re-calibration is required for each lithographic process change—including changes in metrology tools. The Schnitzl focusing fiducials are often used to map out FPD across a stepper or scanner field, but methods similar to those described in “Comprehensive Focus-Overlay-CD Correlation” by Dusa, supra, would need to be implemented to obtain a dynamic focus map. Nevertheless, wafer height variation and scanning dynamics are not considered in the discussion.
Summarizing:
We have described several methods for determining FPD. Common to all of these methods is that a feature (focusing fiducial or FF) is printed on a wafer and the focusing fiducial subsequently measured. The data from the focusing fiducial is processed and an FPD value, δZ, determined. Further, and common to all these methods, the contributions of wafer height, lens aberrations (in the form of lens field curvature), and stage synchronization are not resolved into their distinct components.